1. Field of the Invention
The present invention relates to a liquid crystal display panel of a type having lines of pixels equal in number to the number of frame lines of a video signal.
2. Description of the Prior Art
There is known an active matrix liquid crystal display panel of a type wherein, for example, in order to improve a vertical image resolution, pixels are employed in a number of lines equal to the number of frame lines of a video signal so that an image for odd-numbered lines can be displayed by pixels for the odd-numbered lines while an image for even-numbered lines can be displayed by pixels for the even-numbered lines.
According to the prior art active matrix liquid crystal display panel, unless image contents at the respective pixels are rewritten, the image contents are retained. Accordingly, when the current field of image is to be displayed by means of pixels for the odd-numbered or even-numbered lines, the preceding field of image is displayed by means of pixels for the even-numbered or odd-numbered lines, respectively, and, therefore, the picture being displayed tends to be blemished. Also, since the image contents are rewritten at one frame cycle, flickering tends to occur, making the displayed picture hard to look at.
For example, FIG. 4 illustrates a standard NTSC video signal including odd and even numbered fields corresponding to odd and even numbered lines of a video display. It is standard in video broadcasting to transmit thirty complete pictures, or frames, per second with each frame made up of 525 lines. However, the lines are not scanned from the top of the display to the bottom sequentially. Studies have shown that a display scanned in this manner would appear to flicker. Rather, 262.5 lines are scanned from top to bottom in a first vertical scan followed by a second top to bottom scan that covers the inbetween lines missed in the first scan. This method of first covering the odd-numbered lines and then returning to cover the even-numbered lines is referred to as interlaced scanning. As shown in FIG. 4, the odd-numbered field of a frame is transmitted prior to the even-numbered field. This leads to a problem in liquid crystal panels used to display video signals.
In typical active matrix liquid crystal display panels, a row of pixel elements corresponds to a line of video. Accordingly, during the odd-numbered field of a video frame, video corresponding to the first line is written to pixels in the first row of the matrix, video corresponding to the third line is written to the third row of the matrix and so on down the matrix. During this time the information previously written to the even rows of pixels continues to be displayed by those pixels.
Likewise, during the even-numbered field of a video frame, video corresponding to the second line is written to pixels in the second row of the matrix, video corresponding to the fourth line is written to the fourth row of the matrix and so on down the matrix. During this time the information previously written to the odd rows of pixels continues to be displayed by those pixels.
Since the previous field information continues to be displayed, for part of every frame period the even numbered field of one frame is displayed with the odd numbered field of the next frame. This can lead to blemishes in the displayed image. Also, since the image contents are rewritten at one frame cycle, flickering tends to occur, making the displayed picture hard to look at.
In view of the foregoing, an attempt has been made to provide a system wherein one and the same image is displayed by means of pixels for each neighboring lines while an image for the odd-numbered field and an image for the even-numbered field are displayed having been displaced one line period so that the respective images of the odd-numbered and even-numbered fields can be displayed alternately at one field period with the use of the respective pixels for the odd-numbered and even-numbered lines.
According to such a display system, the image of the current field and the image of the preceding field are simultaneously displayed and, therefore, the displayed image would not be blemished. Also, since the image contents are rewritten for each field period, no flickering would occur.
However, since the same image may be displayed by the pixels in the neighboring two lines, the image displayed tends to be lower in vertical image resolution.
Two different methods have been suggested in U.S. Pat. No. 4,842,371 issued to Yasuda et al. One method disclosed by Yasuda et al. stores the odd numbered field information received from the video signal into memory while scanning the odd rows of the matrix with the odd numbered field information and even rows of the matrix with the even numbered field information stored from the previous frame. Thus lines one and two are updated at the same time, followed by lines three and four, and so on down the display. In the next half cycle, the even numbered field information is stored while the matrix is again fully scanned both with the even numbered rows getting the even numbered field information and with the odd numbered rows getting the odd numbered field information stored from the same frame. This effectively doubles the refresh rate of the display, gives full vertical resolution and effectively reduces flicker. This method would, however, exhibit some of the blemishing mentioned above.
A second method disclosed by Yasuda et al. involves assuming that intermediate points of a scan line can be used to approximate the state of the next row of pixels. Pixel elements for even numbered rows are shifted to fall between pixel elements for odd numbered rows. During the odd numbered field of a video signal the intensity information that is to be written into adjacent pixels in an odd numbered row is averaged and the result written to the pixel that lies between and below them in the next (even numbered) row. Likewise, during the even numbered field of a video signal the intensity information that is to be written into adjacent pixels in an even numbered row is averaged and the result written to the pixel that lies between and below them in the next (odd numbered) row. As in the previous method, this method writes data to each pixel twice in a frame cycle. This effectively doubles the refresh rate of the display, reduces blemishing and effectively reduces flicker. However since, like the first method mentioned, the same image may be displayed by the pixels in two neighboring lines, the image displayed tends to be lower in vertical resolution.